This invention relates to the use of a process tailgas (low specific heating value gas) from the manufacture of carbon black for fueling a reciprocating engine.
Internal combustion reciprocating engines have been used for many years as electric power generation prime movers or directly for supplying industrial power needs. In these applications they are normally fueled by natural gas, diesel fuel or other conventional fuels. In some cases they are also fueled by combustible process gases produced as a byproduct from other manufacturing processes. In these cases the specific heating value of the process tailgas has typically been greater than 150 British thermal units (Btu) per standard cubic foot of gas (scf).
The tailgas produced as a byproduct of carbon black manufacture is typically composed of the following:
While the acetylene and higher hydrocarbons make up less than 1% of the tailgas by volume, they account for nearly 10% of the heating value of the tailgas.
The relative amounts of hydrogen and carbon monoxide can vary with the feedstock used and the carbon black grade produced but the heating value of the tailgas produced will usually be in the range of 50-65 Btu per scf as it leaves the process.
Historically, spark ignited reciprocating engines utilizing gaseous fuels have operated with air/fuel ratios slightly leaner than stoichiometric ratios and have been designed with natural gas as the base fuel. Natural gas typically has a net heating value of approximately 900 Btu per scf. When a fuel with a significantly lower specific heating value is used, the engine is not be able to achieve its rated capacity.
Recently, industrial reciprocating engines have been designed to operate with much leaner fuel mixtures in order to reduce emissions of nitrous oxides. The premise is that by increasing the uncombusted mass flowing through the cylinder, the peak combustion temperature is suppressed which in turn suppresses the formation of the nitrous oxides.
It has been discovered that the tailgas from a carbon black manufacturing process can itself be processed and used as a fuel in an industrial reciprocating engine.
Broadly, the invention embodies the formation and use of a fuel gas for a reciprocating engine, which fuel gas comprises a mixture of a low specific heating value gas mixed with combustion air approaching a stoichiometric ratio. Such a fuel gas can achieve a mass flow suitable for modern reciprocating engine designs and surprisingly has sufficient energy content to allow for near full rated engine capacity. The low specific heating value gas, initially having a heating value of approximately 50-55 Btu/scf, is the primary component in a fuel gas for a reciprocating engine. The low specific heating value gas is conditioned, if necessary, to remove water vapor thereby forming a treated gas with a heating value of approximately 90-110 Btu/scf. The treated gas is mixed with combustion air to form the fuel gas which fuel gas is used in a reciprocating engine. Preferably, the low specific heating value gas comprises hydrogen and carbon monoxide within certain defined percent ranges.
In a preferred embodiment carbon black tailgas is used as the low specific heating value gas. The tailgas can contain as much as 45% water vapor which when reduced to about 5% water vapor increases the specific heating value to the 90-110 Btu per scf range. Because the major variation in tailgas composition among various carbon black grades is due to differences in water content, the heating value of the dewatered tailgas is relatively consistent. The principle components of the tailgas that provide the fuel value are hydrogen and carbon monoxide. Individually these gases are poor candidates for a reciprocating engine fuel. The hydrogen burns too fast and the carbon monoxide burns too slow. The fuel gas of the invention ignites and burns much more readily than carbon monoxide alone while not exhibiting the tendency of hydrogen to burn rapidly with a fast pressure rise (similar to detonation).
In a particularly preferred embodiment of the invention tailgas from a carbon black manufacturing process is dried or dewatered to increase its heating value from about 50-65 Btu/scf to about 90-110 Btu/scf. This dried gas is mixed with combustion air to form the fuel gas. The fuel gas is turbocharged, subsequently cooled and introduced into the engine manifold and ignited. The combustion gases from the engine are used to drive the turbocharger.